This invention relates to printed circuit boards, and more particularly to plated through holes or "vias" used on multi-layer printed circuit boards.
Typically through holes are drilled through conductive layers within the printed circuit board and are used to effect soldered connections between the boards and the devices to be mounted to the boards. These through holes are then plated, usually by electroless plating techniques. This plating results in vertical conductors or "vias" being deposited on the through holes. "Vertical" refers to the direction normal to the top surface of the board. In usual practice, a "pad" is formed on the outer surface of the printed circuit board when each hole is plated. This results in a significant surface area on the outer layer of the board, as shown in FIG. 1. These pads therefore reduce the available area on the outer layer of the printed circuit board. More importantly, these pads must be completely covered with solder in order to avoid a cosmetic defect in the finished product. The pads are also prone to lifting, another defect. For these reasons, the pads have been a major source of rejections of printed circuit boards. It has been found that once the through holes are soldered, the pads do not significantly contribute to the integrity of the printed circuit board's circuitry. The existance of a pad surrounding a through hole allows solder to completely cover the hole without penetrating the hole, thus masking a defective connection.
The pads do have one benefit; that is, they provide an easy place to wick solder into the through hole. In cases where the purpose of the through holes is to provide a via solely for interconnection between layers of the printed circuit board, the wicking capability of the pads has little significance because it is possible to use electroless plating techniques in order to provide enough conductive material to mechanically strengthen the via. In other cases the via is used for effecting connections of circuit components onto the board. In such cases, the ability to wick solder into the via is important because, without the wicking of solder into the via, a circuit component would not be firmly attached to the printed circuit board. Therefore, padless printed circuit boards eliminate some of the problems of circuit board pads at the expense of detracting from the solder wicking characteristics inherent in boards having padded plated through holes.
It should be pointed out that, while such boards are designed without pads, the plating on the through holes sometimes extends on the top or "solder side" of the printed circuit board beyond the outer circumference of the through hole. These holes are nevertheless considered to be not padded as long as the extension of the plating on the exterior surface of the printed circuit board is less than twice the thickness of the plating material itself.